MySQL 5 Storage Engines

June 14, 2005

New Storage Engines in MySQL 5

MySQL 5 offers a number of new storage engines (previously
called table types). In addition to the default MyISAM storage engine, and the InnoDB,
BDB, HEAP and MERGE storage engines, there are four new types: CSV, ARCHIVE,
FEDERATED and EXAMPLE, as well as a new name for the HEAP storage engine. It is
now called the MEMORY storage engine. None of the new types are available by
default - you can check for sure with the SHOW ENGINES statement. Here is what
is on my default version of MySQL Max:

To add
support for the missing storage engines, you currently need to build MySQL with
certain options. It is likely though that there will be binary versions that
include these storage engines by default at some point. Until then, there is no
other way to enable them.

Changes in the MEMORY storage engine

Previously, the HEAP storage engine only made use of hash indexes. These allow
finding specific matches extremely quickly, but do not return any kind of range
data. An index matches a record, but there is no ordering to allow it to return
subsequent records. Only the complete index can be used, the concept of
leftmost prefixing (using the left part of an index) does not apply. The MEMORY
storage engine now permits BTREE indexes as well (the kind used by MyISAM
tables by default).

To specify an index type, use the USING clause, as in the following examples:

The HASH index is still the default, and will be the type of
index created if you do not specify a particular kind.

Now support AUTO_INCREMENT

Now support INSERT DELAYED

Support indexes on columns that can contain NULL values

Never get converted to disk tables. (Temporary internal tables
are automatically converted to disk table if they get too big, MEMORY tables
never are. The max_heap_table_size variable (it hasn't yet changed its
name to reflect the new storage engine name) places a limit on the memory utilization
of MEMORY tables, and you can always place a MAX_ROWS limit as well, when
creating the table.

The EXAMPLE storage engine

Added in MySQL 4.1.3 and only
of interest to developers, the EXAMPLE storage engine does nothing, but is
there to provide simple source code for developers to base new storage engines
on. For those interested, the source code can be found in the sql/examples
directory.

The FEDERATED storage engine

Added in MySQL 5.0.3, to make
use of it you need to use the --with-federated-storage-engine option to
configure when building MySQL. The FEDERATED storage engine allows you to
access data from a table on another database server. That table can make use of
any storage engine. Let's see it in action. First, CREATE a table on a remote
server (you can do this on the same server for testing purposes, but doing so is
fairly pointless otherwise).

CREATE TABLE myisam_table (f1 INT, PRIMARY KEY(f1))
ENGINE=MYISAM;

Assuming that the default is
set to create MyISAM tables (FEDERATED tables can access tables of any type),
the above statement creates a definition file (.frm), an index file (.MYI) and
a data file (.MYD). If you had created an InnoDB file, MySQL would create a
definition (.frm) and index and data file (.idb). Now create the FEDERATED
table on another server. The original table must always exist first:

This creates a definition file (.frm), but the data and
index files are that of the table on the remote server. The only unusual syntax
is the COMMENT, which supplies the username, password (optional), port
(optional), database and table name. This method is not particularly elegant,
or secure, as the password is stored in clear text available to anyone who has
access to the table data. However, in most cases it is likely that whoever has
access to the FEDERATED table can also have access to the remote table, so this
should not be too much of an issue. Bear in mind that this method of connecting
will likely change in a future version.

There are some limitations on the use of FEDERATED tables. They are useful for
easily accessing data on a different server, but fall short in many areas:

They do not support transactions (since the remote server is
contacted once, and the results returned to the local server)

Similarly, there is no way of being sure that the integrity of
the local data is intact.

No prepared statements.

The CSV storage engine

Added in MySQL 4.1.4, tables of
type CSV are actually just comma-delimited text files. This can be quite
useful, and they exist to allow MySQL to interact easily with other
applications that make use of CSV files, such as spreadsheets. They make no use
of any sort of indexing. To enable this storage engine, use the --with-csv-storage-engine
configure option when building MySQL.

Let's see how this works. You can import an existing CSV file. Assume you have
a file containing firstname, surname and age, as follows:

With no indexes, the SELECT is
not at all efficient, and performs a complete table scan. Conversely, you can
INSERT a record from the MySQL client:

mysql> INSERT INTO csv_names VALUES('Quinton','Baxter','75');

and view the change in the CSV
file:

"Jacob","Mbezela","42"
"Schabir","Field","29"
"Quinton","Baxter","75"

The ARCHIVE storage engine

Added in MySQL 4.1.3, the
archive storage engine lives up to its name by storing large amounts of data
without taking up too much space. It too makes no use of any sort of indexing,
and there are no means to repair the table should it become corrupted during a
crash. To enable this storage engine, use the -with-archive-storage-engine
configure option when building MySQL.

This, as always, creates a .frm
definition file, as well as .ARZ and .ARM data and metadata files.

Being an archive, you cannot DELETE, UPDATE or REPLACE records - you can only
INSERT and SELECT. Again, with no indexes, the SELECT needs to perform a
complete table scan. Although the records are compressed upon insertion,
OPTIMIZE TABLE can compress the entire dataset even further. A .ARN file will
temporarily appear when this occurs.

The new storage engines, whilst tricky for most people to use since they
require rebuilding MySQL, can be useful for those with special needs. After
all, people have taken the time to write them! They are bound to be included in
the binaries at some point, so if you are dying to try them, but rebuilding
MySQL is not for you, you hopefully do not have to be patient for long. Even if
you are able to use them now, keep an eye on the official documentation, as
things are likely to change as they approach maturity. Good luck!